Effect of C2HF5 and C3HF7 on methane and propane ignition and laminar flame speed: Experimental and numerical evaluation

After the restriction on several ozone-depleting compounds, including the high efficiency fire suppressant Halon 1301(CF3Br), several alternatives have been proposed. Among them, HFC-227-(C3HF7) and HFC-125 (C2HF5) represent two of the most-used fire suppressants in the industry because of their environmentally favorable properties. Due to their increasing demand, it is very important to understand their combustion properties to optimize their applications and to prevent undesirable events. To this end, the present work examined the effect of C2HF5 and C3HF7 on CH4 and C3H8 laminar flame speeds and ignition delay times. The experimental techniques included freely propagating flames to obtain un-stretched, laminar flame speed and a shock tube for the ignition delay times in fuel-O2-suppressant mixtures highly diluted in Ar (∼98%) using OH* emission near 307 nm. The laminar flame speed experiments were performed at 1 atm over a range of equivalence ratios from 0.7 to 1.3, and the shock-tube tests were done near 1.5 atm over a 1350-2200 K temperature range. A chemical kinetics mechanism was assembled using a HFC set of reactions together with a recently updated C0-C5 hydrocarbon mechanism and OH* chemistry. The results suggest that the tested agents may not be good alternatives as ignition preventers, although they can reduce the laminar flame speed, as a proof that they can be used as fire extinguishers. Comparisons between modeled and experimental data show that the HFC sub-mechanism behaves well, however it can be improved. Surprisingly, a sensitivity analysis shows that many of the top reactions containing fluorinated compounds are classified as ignition-promoters, especially for the experiments with CH4. This work presents some of the first fundamental ignition delay time and flame speed data for HFC-227 and -125, and the results can be used as the basis for future HFC-based chemical kinetics mechanism improvements and to further understand their impact on the combustion process.